One type of conveyor for transporting material is a pipe conveyor, which can be used to protect the material being transported by enclosing it. As such, pipe conveyors are often used in situations where spillage or dust may be an issue or where use of conventional conveyor systems may be too costly or hazardous due to environmental or population concerns. Pipe conveyers may be useful, for example, to convey bulk material between the phases of mining, processing, and storage.
Some pipe conveyors transport material in a circular cross-section formed by overlapping belt edges and using idlers arranged in a hexagonal pattern to form the tubular pipe-like shape. At the loading point, these systems provide a trough or flat conveyor for loading of the material. After loading the material, the belt is formed into a pipe shape for the transport length of the system and re-opened at the destination for the unloading of the material in the standard manner of a troughed or flat conveyor. Because the material is enclosed by the belt during transport, spillage, scattering, pollution, and flying dust may be reduced. These systems also may allow the pipe conveyor to maneuver both vertical and horizontal curves that may be difficult for conventional conveyors to pass through. Also, because pipe conveyors can load and discharge the bulk material in the conventional manner, standard equipment may be used at the head and tail ends.
Pipe conveyors are also useful in situations in which the conveyor layout requires horizontal and/or vertical curves, especially, when the conveyor layout includes a vertical rise or fall. Conventional pipe conveyors, however, are generally limited to being used in conveyor systems with vertical angles of less than 30 degrees as measured from a horizontal axis. While some pipe conveyor designs that allow for pipe conveyors to rise at vertical angles greater than 30 degrees, such systems were proposed more than a quarter of a century ago but have apparently not found successful commercial implementation.
In such conventional systems, the pipe forms a continuous tube. Lifting of the carried material appears to rely on a continuous carry/fall sequence that transports slugs of material for some distance up the tube by means of friction with the pipe walls until the slug collapses and the material falls back down the pipe to a higher level than where it began the previous carry/fall cycle. While prominent pipe conveyor vendors and materials-handling universities have demonstrated in experiments that such a transport mechanism can occur, it appears that difficulties with this approach have, at least to date, prevented such pipe conveyors from being offered for sale.
There are a number of significant difficulties or disadvantages inherent to such systems, which would be manifested if pipe conveyors operating on the continuous-tube principle could be commercialized. One weakness is that the efficiency of transport is likely to be highly dependent on the bulk flow properties of the carried load, which may vary continuously with some partially-processed materials. Another is that—under certain feed conditions—a long void could form below a filled length of pipe, where the filled length is temporarily supported by natural arching or bridging. Such a column of material—having been lifted to a significant height—could then break free, resulting in a sudden collapse of the material in the column, and causing the kind of catastrophic air-blast sometimes associated with hung-up ore passes in underground mines. Yet other limitations of such designs are that special procedures using loose flexible plugs are required to empty the conveyor, and that these designs have only been proposed for conveying in the upwards direction.
Another significant limitation of steep or vertical pipe conveyor systems is that the tensile capacity of the pipe belt increases only in direct proportion to the width of the belt. However, the weight of the material enclosed within the tube and supported by the tensile capacity of the pipe belt increases roughly in proportion to the square of the width of the belt. Therefore, as a designer of a steep or vertical pipe conveyor attempts to increase tonnage by increasing the effective diameter of the carrying tube, it may be found that the tensile capacity of the pipe belt quickly becomes the limiting constraint. Even when intermediate traction drives are provided, this adverse relationship may result in the intermediate drives having to be spaced much more closely than would otherwise have been necessary.
Another possible disadvantage of such steep or vertical pipe conveyor systems is that they may be prone to an unacceptable amount of leakage of carried material fines between the overlapping flaps of the pipe belt. Since the material in the tube is continuously displaced by the carry/fall transport mechanism, there is less opportunity for material to cake at and seal the junction between the overlapping pipe belt flaps.
In another type of steep or vertical pipe conveyor system, a tension element separate from the pipe belt, such as a chain or cable, carries a series of stiff diaphragms within the tube formed by the pipe belt. The diaphragms are configured to stand in planes perpendicular to the main axis of the tube, and to carry most of the weight of the material in steep sections of the pipe conveyor, and transfer the weight of that material to the tension element. The tension element in turn is supported by a drive means at a location beyond the conveyor discharge point. The purposes of such a configuration are apparently to lift discrete volumes of material carried by the discs, and to relieve the pipe belt of the tension-carrying role, so that the belt's primary role becomes that of enclosing the material. A major limitation of such a design is that, for dense materials or high lifts, the tension accrued by the tension element quickly rises to exceed its tensile capacity. Therefore the approach becomes impractical for the high lifts encountered in some applications such as mining. Another significant disadvantage of such systems is that the tension element is in direct contact with the load so that if the load is an abrasive and somewhat sticky material, such as a moist ore, a high rate of wear would occur on the tension element and on the means used to drive it, such as a sprocket in the case where the tension element is a chain. Yet another disadvantage of such an approach is that the rigid discs described are susceptible to damage by larger lumps of ore, as is the tension chain.